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Ophthalmology => Eye => Anthropod
Anthropod
INTRODUCTION
Arthropod, animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects, such as ants, beetles, and butterflies; crustaceans, such as lobsters, shrimps, and crabs; and arachnids, including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth. More than one million arthropod species have been identified-more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined. This figure is considered a low estimate of the phylum's actual size because many arthropod species have yet to be discovered and documented. Some scientists suggest the number of arthropod species in tropical forests alone may approach six million to nine million.
Arthropods have adapted to life on land, at sea, and in the air. They occupy an array of habitats, from scorching deserts and scalding hot springs to snow-capped mountains and frigid fjords. As plant pollinators, nutrient recyclers, and prey for other animals, they are essential members of the web of life. If all arthropods suddenly were to perish, thousands of animals and plants that depend on their services soon would vanish as well.
Many arthropods, including shrimp, lobsters, and crabs, are harvested as food for people throughout the world. Other arthropods provide the ingredients for fabric dyes, wood preservatives, and medicines. Members of one arthropod class in particular-insects-can be formidable pests, devouring crops, destroying wood structures, and spreading malaria and other life-threatening disease.
PHYSICAL CHARACTERISTICS
Most arthropods are small, and many are microscopic. Several species are quite large: a fully grown coconut crab can weigh 4 kg (9 lb), about the size of a small cat. Another giant is the Alaskan king crab of the northern Pacific Ocean, whose outstretched claws may span 150 cm (60 in).
Lacking internal skeletons, arthropods wear their "bones" on the outside in the form of an armored exoskeleton. This durable shell is made of chitin, a hard material containing cellulose and further strengthened by protein and, in some species, calcium carbonate. The exoskeleton provides many protective advantages, including protection from dehydration, but it also presents some drawbacks. For example, an exoskeleton has to be shed, or molted, if growth is to occur, and during this process, the animal is soft and vulnerable. The strength of the exoskeleton diminishes as an animal grows, and terrestrial arthropods, which lack buoyant support from water, do not grow very large.
The exoskeleton covers the arthropod's entire body, even the eyes, and is jointed like a medieval knight's armor to facilitate movement of the limbs. Arthropod limbs are adapted for many forms of locomotion-leaping, swimming, walking, scurrying, and burrowing. Legs of land-dwelling arthropods may be tipped with hooklike claws, hairs, or sticky pads for gripping, clinging, or adhering to various surfaces. Among many groups of arthropods, modified limbs serve as mouthparts that are well suited for collecting and handling food.
Arthropods have evolved a variety of strategies for supplying oxygen to their cells. Many small and primitive arthropods absorb all the oxygen that they need through their exoskeleton. Among insects, centipedes, and millipedes, metabolic oxygen is obtained from a convoluted system of tracheae, tiny tubes connected to air holes, known as spiracles, in the exoskeleton. Aquatic arthropods have gills with which they obtain dissolved oxygen from their watery surroundings. The terrestrial arachnids have book lungs, modified gills that consist of a series of parallel folds of the exoskeleton arranged like the pages of a book. Air circulates between the folds, exchanging gases with the blood that circulates within the folds.
Arthropods possess an open circulatory system in which the heart forces blood through the body cavity to bathe internal tissues with nutrients from the digestive system. The blood of large spiders and crabs is tinged a bright blue by the oxygen-carrying pigment hemocyanin. In insects the tracheae supply oxygen directly to cells, so insect blood lacks oxygen-carrying pigments and is colorless.
An arthropod's brain consists of a ring of large nerve bundles, called ganglia, attached to a ventral nerve cord, a chain of smaller ganglia connected by giant nerve fibers. In the case of the cockroach, one of the most extensively studied arthropods, these giant fibers carry impulses 10 times faster than ordinary nerves, traveling the length of the ventral nerve cord in about 0.003 seconds. Arthropods sense the environment using a variety of sensory organs, including simple light-sensitive eyespots, simple eyes, multifaceted compound eyes, antennae, abdominal bristles, and receptors on leg hairs.
ARTHROPOD TYPES
Traditionally, living arthropods are classified among three subphyla: the Chelicerata, Crustacea, and Unirama. Trilobites, a fourth subphylum, flourished during the Cambrian and Ordovician periods of the Paleozoic Era (570 million to 435 million years ago) but became extinct by the mid-Permian Period (about 250 million years ago), for reasons that are not fully understood.
Chelicerata, named for their chelicerae, or specialized mouthparts, include scorpions, spiders, ticks, mites, and horseshoe crabs. These creatures have six pairs of appendages, one or two body regions, and lack wings and antennae. Most live on land, although a few are aquatic.
With a few exceptions, the Crustacea are aquatic, occupying both freshwater and saltwater niches. Familiar crustaceans include shrimps, lobsters, and crabs. All crustaceans have ten limbs, three body regions, and two pairs of antennae. Many of the smallest crustaceans are zooplankton-drifting or weak-swimming animals that form the base of most marine food webs.
The Unirama include centipedes, millipedes, and insects. Centipedes have wingless, multisegmented bodies, with one pair of legs on each body segment. The bodies of millipedes are also wingless and multisegmented but have two pairs of legs per segment. Insects have three pairs of legs, one pair of antennae, and may be wingless or have one or two pairs of wings. The majority of insects are land-dwellers, although some live in freshwater or marine environments.
LIFE CYCLE AND REPRODUCTION
In most instances, both sexes of arthropods occur separately. Males commonly pass sperm to females in sealed packets called spermatophores. The males lay these packets on the ground, and the females later pick them up, or the male deposits them into the female's genital opening. Among crustaceans, millipedes, spiders, mites and some insects, males transfer sperm directly to females.
Fertilized arthropod eggs hatch after days, weeks, months, and even years. Most species deposit their eggs externally, but some species hatch their eggs internally, bearing live young. In many species, such as spiders, the hatchlings look like miniature adults. The larvae of other arthropod species have little or no physical resemblance to adults. For example, caterpillars are quite distinct from the butterflies that they become as adults. These larvae, in the process of metamorphosis, pass through a series of distinct phases to become adults. Larvae may also inhabit different environments and eat different foods than their parents. The life spans of arthropods range from a few weeks to several decades.
ARTHROPOD EVOLUTION
The evolutionary origins of modern arthropods are unclear and complex. It is generally accepted that the phylum is polyphyletic-that is, derived from several separate ancestral lines. The ancestors of arthropods were ancient aquatic segmented worms, similar to present-day annelids, although the fossil evidence is sketchy. The ancient seas of the Cambrian and Ordovician periods (570 million to 435 million years ago) were teeming with aquatic arthropods, especially trilobites.
During the Silurian and Devonian periods (435 million to 345 million years ago) the arthropods, along with amphibians, were the first animals to leave the sea and colonize the land. When they emerged from the sea, they had few, if any, competitors. They swiftly adapted to the demands of terrestrial life, occupying new niches as predators, plant eaters, parasites, and decomposers. The earliest terrestrial arthropod fossil is of a scorpion-like arachnid. The earliest insect fossils come from a few million years later. These early terrestrial arthropods dominated the land more than 100 million years before the arrival of the first dinosaurs.
Scientific classification: Arthropods make up the phylum Arthropoda, which is divided into three living subphyla: Chelicerata, Crustacea, and Unirama.
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